1. Field of the Invention
This invention relates to devices capable of sensing and indicating when a predetermined minimum temperature has been reached. The invention has particular utility in connection with detecting when food such as ground beef has been cooked to a safe temperature for consumption, and will be described in connection with such utility, although other utilities are contemplated.
2. Background of the Invention
E. coli is found in the intestines of all mammals. E. coli contamination of food causes tens of thousands of cases of food poisoning annually in the United States. A particularly virulent strain E. coli 0157:H7 was found to be responsible for several widely publicized outbreaks of food poisoning during the first half of 1993, including an outbreak in Washington state which caused three fatalities, and fatal outbreaks in Illinois, Indiana, Michigan and Maine. Recent U.S. Public Health Service purchase studies suggest that two to three percent of all hamburger sold in the United States contains E. coli 0157:H7. Thus, this organism presents a serious food-borne illness problem.
The problem of E. coli contamination and particularly E. coli 0157:H7 contamination is particularly severe with ground meat. The inside of a cut of beef is relatively aseptic. The foreign organisms are contained primarily on the surface of the meat. This means that cooked cuts such as steaks and roasts are relatively safe since the outside surface of the meat is directly exposed to the cooking heat. However, since in grinding meat, the former external surface of the meat becomes dispersed throughout the bulk of the meat, with the result that any contamination on the external surface of the meat may be spread throughout the meat. To kill the organisms, the cooking heat must reach throughout the entire bulk of the ground meat.
Proper cooking can kill harmful organisms such as E. coli, Salmonella, Listeria and other harmful organisms. In the case of E. coli 0157:H7, the United States Department of Agriculture recommends that ground beef products be cook to an internal temperature of 155.degree.-160.degree. F. It is difficult to determine when this condition has been met.
Conventional liquid-glass thermometers and/or dial stick cooking thermometers generally cannot readily be used for measuring the internal temperature of a ground beef patty due to the geometry of the probe. Moreover, such thermometers have relatively slow reaction times, and have relatively high heat capacity which may contribute to considerable inaccuracies caused by introduction of the probe into the patty. Moreover, the portions of the probes extending external from the patty may be subjected to cooking heat which may further contribute to inaccurate readings. Liquid glass thermometers also pose a particular safety hazard in the event of breakage. Additionally, neither liquid/glass thermometers nor dial stick thermometers provide a permanent record, which may be important for commercial users such as fast food restaurants or the like.
Thermocouple and thermistor devices also can be used to measure temperature of ground beef patties. However, thermocouple and thermistor devices are relatively expensive and require expensive, fragile electronic equipment for calibration and measurement. Also, thermistor devices can be relatively slow acting and have been observed to require twenty seconds or more to achieve stable temperature readings.
Another problem with current instruments of measuring temperature is that they must be used many times to be economical. Unless very carefully cleaned after each use, the temperature measuring device can actually spread contamination from one piece of meat to another.
It is thus an object of the present invention to provide a temperature sensing and indicating device which overcomes the aforesaid and other problems of the prior art. A more specific object of the present invention is to provide a low cost, temperature sensor and indicator device which advantageously may be used both commercially and domestically, for determining when a ground meat patty or other food product has been cooked sufficiently to raise the internal temperature so that it is safe for human consumption.